High-speed inverter pulsing focuses and constricts the arc for faster travel speeds at reduced amperage levels, which narrows the heat affected zone and lowers heat input, helping to reduce distortion. This results from rapidly switching the current between the peak and background amperage — at frequencies ranging from 100 to 5,000 hz (compared to only 1-20 hz with older, conventional technology). Pulsing at such a high rate doesn’t give the arc enough time to fully expand to its maximum width before the current is switched to the background setting.
The results of high frequency pulsing are profound for the heat sensitive alloys and stringent weld requirements in aircraft repair. Reduced amperages control heat input for a better as-welded microstructure with reduced distortion. Added benefits include penetration control, preventing burn-through and reduced cracking. The focused arc allows the operator to more accurately place the weld, control bead width and heat affected zone, reducing the chance for the most common causes of weld rejection.
Alternating current and advanced waveforms
AC is commonly used for welding aluminum and magnesium components. As with DC, the width of the arc is determined by welding amperage, tungsten diameter, and electrode preparation. Aluminum and magnesium are challenging to weld. While AC provides some key benefits for these materials, it’s not without issues.
As we look at aluminum, it’s important to note that formation of a high-melting surface oxide is a trait that separates this material from other alloys. All metals form oxides, but in most cases they melt at a lower temperature than the base material. This is not true for aluminum, as the invisible surface oxides melt at approximately three times the temperature of the base metal. Therefore, the oxides must be removed for welding.
AC balance control describes the ability to adjust the ratio of time spent in electrode negative (EN) compared to electrode positive (EP) in the cycle. Inverters allow much higher EN settings (up to 99 percent) compared to conventional maximums of approximately 70 percent. The balance can be adjusted to provide adequate arc cleaning without the severe balling action produced by excessive EP time in the cycle. This reduction in balling action reduces arc wander, pulls in the etching zone and directs more heat into the work for faster welding speeds. In addition, sharpened alloy tungsten electrodes can be used for welding aluminum and magnesium to provide better arc starts and control.
The effects of AC balance are illustrated by the extremes of EN percentage in the AC wave. Very little EN results in greater cleaning action, shallow penetration (a benefit on thinner materials), and severe electrode balling because most of the heat is directed up, onto the tungsten. At the other end of the spectrum, a high amount of EN places most of the heat into the work. Cleaning is minimized along with the balling action of the electrode. This extended balance range allows the arc to be fine-tuned according to base metal conditions in each application. It achieves greater penetration, faster travel speeds, narrows the weld bead, extends tungsten life, produces a smaller etched zone, and permits the use of a smaller diameter tungsten to more precisely direct the heat into the joint.
AC frequency control is a newer adjustment made possible by inverter technology. Older TIG technology is typically locked in at the line frequency: 60 hz in North America and 50 hz in other locations around the globe. New inverter technology gives operators the ability to dial that in between 20 hz and 400 hz. Current alternating at 60 hz produces a relatively wide, lazy arc that lacks directional control. This can be seen in a typical T-joint application where the arc wanders between the toes of the weld and lacks the focus to drive into the corner.
The ability to increase the AC frequency has a dramatic effect on the arc characteristics as well as the weld bead. Higher frequencies of alternation limit the time that the arc expands on each half-cycle. This creates a narrow, focused arc that has significantly better directional control, helping the arc to reach the throat area of a weld and ensuring adequate penetration. Arc wander is virtually eliminated, which saves important features in aircraft components that may otherwise be affected. The improved directional control of higher AC frequencies is also very helpful when welding sections of dissimilar thickness.
Miller FABTECH Presentation Highlights Advanced GTAW Technology for Welding Repair on Alloys Used in Aircraft Engines, Components
Brent Williams will present "Inverter Technology Improves Aircraft Engine and Component Repair" on Wednesday, Nov. 3 at 4:05 p.m. at the FABTECH show in Atlanta.
Back to the basics By Joe Escobar Welding is used in many aircraft applications. One of the most popular methods is gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG...